Agree with previous reviews that the DB obscures part of the view but there are still grand vistas taking the Tiergarten, Brandenberg Gate, Sony Center and Potsdamer Platz.
The entry fee was €5.80 and you get taken up 24 floors in 20 seconds - I gather it's the fastest lift in Europe. The observation deck is on 2 levels. The first level has a café and there are storyboards in both English and German giving a fascinating history of Potsdamer Platz starting in the 1830s through the Cold War and up to the early 1990s when Potsdamer Platz was transformer from a waste ground into the shiny new collection of skyscrapers we see today. The second level gives you the full 360 view from the building though unfortunately the DB blocks part of the skyline.
Overall, I thought this attraction was good value for money. There were no huge queues and for my the best thing was reading all about the history and seeing all the photos of how Potsdamer Platz looked through its changing history. There could well be better observation attractions around Berlin but it's worth a visit if you're this part of Berlin.